Plastisols comprise a dispersed phase that includes but is not limited to finely divided particles of a non-crosslinked thermoplastic organic polymer and a liquid phase comprising a plasticizer for the polymer. Some texts on the subject define plastisols as including any organic polymer as the dispersed phase, while others limit the polymer to homo- and copolymers of vinyl chloride. The description of the present invention employs the broader definition.
Major end use applications of plastisols are as films, coatings and molding materials. Films and coatings are prepared by applying the plastisol to a surface as a flowable liquid. The layer of plastisol is then heated to evaporate any volatile liquids and fuse the particles of polymer to form a solid layer.
Plasticizers can be categorized based on their ability to solvate the dispersed polymer. Certain esters of phthalic acid, such as the mixed butylbenzyl ester (BBP) and benzoates of glycols such as diethylene and dipropylene glycols are particularly good solvators for polyvinyl chloride. The benzoates are therefore preferred for use in plastisol formulations containing this polymer that will be subjected to rapid processing. For many end use applications the high viscosity imparted by solvating plasticizers requires use of liquid hydrocarbons, ketones, or other classes of organic liquids to achieve the desired processing viscosity. The resultant plastisols are referred to as organosols and the organic liquids as diluents. The concentration of organic liquid in an organosol is typically greater than 5 weight percent, based on the total weight of the plastisol.
Plastisols containing other diesters of phthalic acid such as bis(2-ethylhexyl phthalate), a moderate solvating plasticizer for PVC referred to in the literature as DOP, are less likely to exhibit the substantially increased viscosity associated with the higher solvating plasticizers mentioned hereinbefore, but may still require use of an organic liquid or other viscosity suppressant(s).
Chapter 26 in volume III of a text entitled Encyclopedia of PVC Technology discusses the use of liquid hydrocarbon diluents as rheology control agents in PVC plastisols and organosols. The hydrocarbons evaluated included aliphatic hydrocarbons, and mixtures of aliphatic with 13, 16 and 98 volume percent of aromatic hydrocarbons. The data in this chapter demonstrate that the viscosity of the plastisol increased from 2.0 to 3.3 Pascal seconds (Pa·s) with increasing aromatic hydrocarbon content of from 0.02 to 98 percent. Both of these viscosity values are within the processable range.
Low viscosity plastisols prepared by mechanically dispersing a paste consisting essentially of finely divided particles of polyvinyl chloride in a plasticizer are described in U.S. Pat. No. 4,950,702, which is incorporated herein by reference. The low viscosity exhibited by the resultant plastisol results from use of either dipropylene glycol monomethyl ether benzoate or tripropylene glycol monomethyl ether benzoate as the plasticizer. The viscosity of the plastisols are sufficiently low that they do not require use of an organic liquid to facilitate fabrication of slush molded articles or coating of the plastisol on to a self-supporting substrate such as a resilient flooring construct.
The effects of various solvents on dilatancy in concentrated phthalate ester plasticized emulsions of polyvinyl chloride were investigated by S. J. Wiley and C. W. Macosko and the results reported in the Journal of Rheology (Volume 26, Issue 6, pp 557-564). The effect of specific solvents on the strength of interparticle interactions was interpreted in terms of steric stabilization theory. The flow strength required to effect dilatancy decreased as the solvent quality of the continuous phase was reduced.
The prior art contains a variety of mathematical formulae for predicting the compatibility of polymers with various liquids, including plasticizers. The term “solubility parameter” was first used by Joel H. Hildebrand to quantify the solvating ability of specific solvents. Hildebrand expressed this parameter as the square root of the cohesive energy density, c, which had previously been defined as c=(ΔH−RT)/Vm. The solubility parameters for numerous organic compounds have been calculated using the formula developed by Hildebrand.
It was subsequently discovered that pairs of polar and non-polar molecules, which should be compatible on the basis of similar solubility parameter values calculated using Hildebrand's formula, were in fact immiscible. The presence of polar groups and the possibility of hydrogen bonding between molecules affected the solubility parameter of a molecule. Hansen considered both of these types of intermolecular interactions in developing his formula in which the total solubility parameter of a molecule is equal to the square root of the sums of the dipole, polar and hydrogen bonding contributions to the solubility of the molecule.
P. Teas developed a method for depicting the contributions of the three types of intermolecular interactions described by Hansen in two dimensions using a triangular graph. Scales for each of the three interactions appear on the three sides of the triangle When the magnitudes of the three types of Hansen interactions are determined for a particular organic liquid, the values are plotted as a single point on the graph. A group of suitable solvents for the polymer can be defined by experimentally determining which members of a groups of organic liquids exhibiting similar solubility parameter values, as represented by adjacent points on the graph, actually swell the polymers and encircling the points representing these liquids.
P. A. Small developed a method for determining the solubility parameter of compounds, including plasticizers and polymers,
by dividing 1) the sum of the molar attraction constants, also referred to as “Small's constants”, exhibited by the functional groups in the compound by 2) the molar volume of the compound. Small's work is reported in the Journal of Applied Chemistry, 3, 76-80 (1953), which is hereby incorporated by reference. Unless otherwise indicated, all solubility parameter values in the following sections are expressed in Hildebrand units. The Small's constants for many solvents are reported in literature issued by suppliers of these solvents.